Nonpolar III-nitride light emitting diodes with long wavelength emission

ABSTRACT

A III-nitride film, grown on an m-plane substrate, includes multiple quantum wells (MQWs) with a barrier thickness of 27.5 nm or greater and a well thickness of 8 nm or greater. An emission wavelength can be controlled by selecting the barrier thickness of the MQWs. Device fabricated using the III-nitride film include nonpolar III-nitride light emitting diodes (LEDs) with a long wavelength emission.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. Section 120 of U.S.Utility patent application Ser. No. 12/189,038, filed on Aug. 8, 2008,by Hisashi Yamada, Kenji Iso, and Shuji Nakamura, entitled “NONPOLARIII-NITRIDE LIGHT EMITTING DIODES WITH LONG WAVELENGTH EMISSION”, whichapplication claims the benefit under 35 U.S.C. Section 119(e) of U.S.Provisional Patent Application Ser. No. 60/954,770, filed on Aug. 8,2007, by Hisashi Yamada, Kenji Iso, and Shuji Nakamura, entitled“NONPOLAR III-NITRIDE LIGHT EMITTING DIODES WITH LONG WAVELENGTHEMISSION”, which applications are incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned U.S. patent applications:

-   U.S. Utility application Ser. No. 12/140,096, filed on Jun. 16,    2008, by Asako Hirai, Zhongyuan Jia, Makoto Saito, Hisashi Yamada,    Kenji Iso, Steven P. DenBaars, Shuji Nakamura, and James S. Speck,    entitled “PLANAR NONPOLAR M-PLANE GROUP III NITRIDE FILMS GROWN ON    MISCUT SUBSTRATES,”, which application claims the benefit of U.S.    Provisional Application Ser. No. 60/944,206, filed on Jun. 15, 2007,    by Asako Hirai, Zhongyuan Jia, Makoto Saito, Hisashi Yamada, Kenji    Iso, Steven P. DenBaars, Shuji Nakamura, and James S. Speck,    entitled “PLANAR NONPOLAR M-PLANE GROUP III NITRIDE FILMS GROWN ON    MISCUT SUBSTRATES,”; and-   U.S. Utility application Ser. No. 12/189,026, filed on Aug. 8, 2008,    by Kenji Iso, Hisashi Yamada, Makoto Saito, Asako Hirai, Steven P.    DenBaars, James S. Speck, and Shuji Nakamura, entitled “PLANAR    NONPOLAR M-PLANE GROUP III-NITRIDE FILMS GROWN ON MISCUT    SUBSTRATES,”, which application claims the benefit of U.S.    Provisional Application Ser. No. 60/954,744 filed on Aug. 8, 2007 by    Kenji Iso, Hisashi Yamada, Makoto Saito, Asako Hirai, Steven P.    DenBaars, James S. Speck, and Shuji Nakamura, entitled “PLANAR    NONPOLAR M-PLANE GROUP III-NITRIDE FILMS GROWN ON MISCUT    SUBSTRATES,” and U.S. Provisional Application Ser. No. 60/954,767    filed on Aug. 8, 2007, by Hisashi Yamada, Kenji Iso, Makoto Saito,    Asako Hirai, Steven P. DenBaars, James S. Speck, and Shuji Nakamura,    entitled “III-NITRIDE FILMS GROWN ON MISCUT SUBSTRATES,”;-   all of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to InGaN/GaN light emitting diodes (LEDs) andlaser diodes (LDs), and more particularly to nonpolar III-nitride LEDsin which the wavelength can be controlled by selecting the barrierthickness of multiple quantum wells (MQWs).

2. Description of the Related Art

The usefulness of gallium nitride (GaN) and its ternary and quaternarycompounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN) hasbeen well established for fabrication of visible and ultravioletoptoelectronic devices and high-power electronic devices. Thesecompounds are referred to herein as Group III nitrides, or III-nitrides,or just nitrides, or by the nomenclature (Al, B, Ga, In)N. Devices madefrom these compounds are typically grown epitaxially using growthtechniques including molecular beam epitaxy (MBE), metalorganic chemicalvapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).

Current nitride technology for electronic and optoelectronic devicesemploys nitride films grown along the polar c-direction. However,conventional c-plane quantum well structures in III-nitride basedoptoelectronic and electronic devices suffer from the undesirablequantum-confined Stark effect (QCSE), due to the existence of strongpiezoelectric and spontaneous polarizations. The strong built-inelectric fields along the c-direction cause spatial separation ofelectrons and holes that in turn give rise to restricted carrierrecombination efficiency, reduced oscillator strength, and red-shiftedemission.

One approach to eliminating the spontaneous and piezoelectricpolarization effects in Group-III nitride optoelectronic devices is togrow the devices on nonpolar planes of the crystal. For example, in GaNcrystals, such planes contain equal numbers of Ga and N atoms and arecharge-neutral. Furthermore, subsequent nonpolar layers are equivalentto one another so the bulk crystal will not be polarized along thegrowth direction. Two such families of symmetry-equivalent nonpolarplanes in GaN are the {11-20} family, known collectively as a-planes,and the {10-10} family, known collectively as m-planes.

The other cause of polarization is piezoelectric polarization. Thisoccurs when the material experiences a compressive or tensile strain, ascan occur when (Al, B, Ga, In)N layers of dissimilar composition (andtherefore different lattice constants) are grown in a nitrideheterostructure. For example, a thin AlGaN layer on a GaN template willhave in-plane tensile strain, and a thin InGaN layer on a GaN templatewill have in-plane compressive strain, both due to lattice matching tothe GaN. Therefore, for an InGaN quantum well on GaN, the piezoelectricpolarization will point in the opposite direction than that of thespontaneous polarization of the InGaN and GaN. For an AlGaN layerlattice matched to GaN, the piezoelectric polarization will point in thesame direction as that of the spontaneous polarization of the AlGaN andGaN.

The advantage of using nonpolar planes over c-plane nitrides is that thetotal polarization will be reduced. There may even be zero polarizationfor specific alloy compositions on specific planes. Such scenarios willbe discussed in detail in future scientific papers. The important pointis that the polarization will be reduced compared to that of c-planenitride structures.

Although high performance optoelectronic devices on nonpolar on-axism-plane GaN have been demonstrated, it is difficult to obtain longwavelength emission from InGaN/GaN MQWs grown on m-plane GaN. This isprobably due to the low In incorporation of the InGaN/GaN MQWs. Theemission wavelength of devices grown on m-plane is typically 400 nm,while the emission wavelength of devices grown on c-plane is 450 nm atthe same growth condition. Reducing the growth temperature increases theIn incorporation; however, crystal quality would be degraded. This wouldbe a significant problem for applications such as blue, green, yellow,and white LEDs.

The present invention describes a technique for the growth of nonpolarIII-nitride light emitting devices in which the emission wavelength fromthe devices can be controlled by the barrier thickness of the MQWs inthe devices. For example, the present invention has obtained blue andgreen emission without the effect of polarization.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa III-nitride film grown on an m-plane substrate, wherein a barrierthickness of the MQWs in the film is 27.5 nm or greater.

Specifically, the present invention discloses a III-nitride filmcomprised of one or more indium-containing quantum wells with a barrierthickness of 27.5 nm or greater. The quantum wells may be sandwichedbetween at least a first barrier and a second barrier, wherein the firstbarrier and second barrier each have a thickness of 27.5 nm or greater.Moreover, the quantum wells' thickness may be 8 nm or greater.

The quantum wells may be nonpolar. In addition, the quantum wells emitlight, having a peak wavelength, in response to a range of injectioncurrents passing through the film; and the quantum wells may have anonpolar orientation, such that interfaces between the quantum wells andthe barrier thickness, and between the quantum wells, are nonpolarplanes, wherein the nonpolar planes and an alloy composition of anactive layer are selected to reduce a polarization of the film orquantum wells, so that the peak wavelength remains significantlyconstant for a range of injection currents, thereby defining a degree ofnonpolarity of the quantum wells or the film. The range of injectioncurrents may produce a range of intensities for the emitted light,wherein a maximum intensity is at least 37 times a minimum intensity.

The quantum wells may comprise two or more quantum wells, namely MQWs.The film may be grown on m-plane GaN. The quantum wells may be InGaNquantum wells and the barrier may be a GaN barrier.

The barrier thickness may be selected to obtain light emission from thequantum wells with a peak wavelength longer than 475 nm, and the lightemission may result from electron-hole pair recombination between anelectron in a quantum well state in a conduction band of the III-nitridefilm and a hole in quantum well state in a valence band of theIII-nitride film. Increasing the barrier thickness increases an emissionwavelength.

In addition, the present invention discloses a device fabricated usingthe III-nitride film as described above. For example, the film may be anactive layer of a light emitting device.

Finally, the present invention discloses a method of emitting light froma III-nitride film, comprising emitting the light from one or moreindium-containing quantum wells with a barrier thickness of 27.5 nm orgreater, and a method of fabricating a Group-III nitride film,comprising growing one or more indium-containing quantum wells with abarrier thickness of 27.5 nm or greater.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1( a) is a cross sectional schematic of the film of the presentinvention, and FIG. 1( b) is a band diagram through the layers of thefilm depicted in FIG. 1( a).

FIG. 2 is a graph plotting wavelength emitted by quantum wells vs.barrier thickness of the quantum wells.

FIG. 3 shows electroluminescence (EL) spectra of the LED with thebarrier thickness of 37.5 nm, for different injection currents, whereinthe spectra are, from bottom to top, for injection currents of 1 mA, 2mA, 5 mA, 10 mA, 20 mA, 30 mA, 40 mA, 50 mA, 60 mA, 70 mA, 80 mA, 90 mAand 100 mA.

FIG. 4 is a graph of peak emission wavelength from a quantum well vs.current.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The present invention describes a Group-III nitride film grown on anm-plane substrate in which the emission wavelengths from the film can becontrolled by selecting a barrier thickness of the MQWs in the film.

Current nitride devices are typically grown in the polar [0001]c-direction, which results in charge separation along the primaryconduction direction in vertical devices. The resulting polarizationfields are detrimental to the performance of current state of the artoptoelectronic devices.

Growth of these devices along a nonpolar direction has improved deviceperformance significantly by reducing built-in electric fields along theconduction direction. However, the emission wavelength of the LEDs grownon on-axis m-plane is typically 400 nm, which is limited to applicationsfor optical devices.

The novel feature of this invention is that III-nitride films emittinglong wavelengths can be grown using thick barrier MQWs. As evidence ofthis, the present invention has grown InGaN/GaN-based LEDs on m-planesubstrates with various barrier thicknesses. The emission wavelength ofthe film grown with a barrier thickness of 12.5 nm was 439 nm, while theemission wavelength of the film grown with a barrier thickness of 27.5nm or greater was 480 nm.

Technical Description

The present invention comprises III-nitride films with a long emissionwavelength utilizing a thick barrier of the MQWs in the growth process.In one embodiment, the barrier thickness is 27.5 nm or greater to obtainlong wavelength emission from the MQWs.

FIG. 1( a) illustrates a III-nitride film 100 comprising one or morequantum wells 102 with a barrier layer 104 a, 104 b thickness 106 a, 106b of 27.5 nm or greater. The quantum wells 102 are typically indiumcontaining III-nitride quantum wells. The quantum well 102 is typicallysandwiched between a first barrier 104 a and a second barrier 104 b,wherein the first barrier 104 a and second barrier 104 b each have thebarrier thickness 106 a, 106 b of 27.5 nm or greater. The film 100 maycomprise quantum wells 102, first barrier 104 a having thickness 106 a,and second barrier 104 b having thickness 106 b.

The quantum well's 102 thickness 108 may be 8 nm or greater. The film'squantum wells 102 may be nonpolar. The film 100 may comprise two or morequantum wells 102, thereby forming MQW's, wherein each of the quantumwells 102 is clad by barrier layers 104 a, 104 b (e.g., a MQW maycomprise multiple stacked quantum well periods, wherein a quantum wellperiod comprises a quantum well 102 clad by barriers 104 a, 104 b). Agrowth substrate for the film 100 may be m-plane GaN 110. The film 100may comprise further layers, such as an n-type layer between the barrier104 b and the substrate 110, and a p-type layer on the barrier layer 104a, in order to fabricate a device such as an LED. The quantum well 102may be an InGaN quantum well and the barriers 104 a, 104 b may be GaNbarriers.

FIG. 1( b) is a schematic of the film's 100 band diagram as a functionof position, from top to bottom, through the layers 104 a, 102 and 104b, showing the conduction band comprising a quantum well energy 112 a ofthe quantum well 102, a valence band comprising a quantum well energy112 b of the quantum well 102, a conduction band comprising a firstbarrier energy 114 a of the first barrier 104 a, a valence band energycomprising a first barrier energy 114 b of the first barrier 104 a, aconduction band comprising a second barrier energy 116 a of the secondbarrier 104 b, and a valence band energy comprising a second barrierenergy 116 b of the second barrier 104 b. The barrier thickness 106 a,106 b may be selected to obtain light emission from the quantum wells102 with a peak wavelength longer than 400 nm or longer than 475 nm, forexample.

FIG. 1( b) shows how the light emission 118 may result fromelectron-hole pair recombination 120 between an electron 122 in aquantum well state such as 124 a, 124 b in the conduction band of theIII-nitride film 100 and a hole 126 in quantum well state such as 128 a,128 b in the valence band of the III-nitride film 100.

In the present invention, the LED device structure's epitaxial layerswere grown using a conventional MOCVD method on a freestanding m-planeGaN substrate. The LED structure was comprised of a 5 μm-thick Si-dopedGaN layer, 6-periods of a GaN/InGaN MQW, a 15 nm-thick undopedAl_(0.15)Ga_(0.85)N layer, and 0.3 μm-thick Mg-doped GaN. The MQWcomprised 8 nm thick InGaN wells. The GaN barrier thickness was variedfrom 12.5 nm to 42.5 nm. After the crystal growth of the LED structure,the samples were annealed for p-type activation and subsequently an n-and p-type metallization process was performed. The p-contact had adiameter of 300 μm and the emission properties were measured at roomtemperature. The measurement was performed at forward currents between 1mA and 100 mA (DC), at room temperature.

Experimental Results

The EL peak wavelength of the LEDs at 20 mA as a function of the barrierthickness is shown in FIG. 2. The measurement was performed at a forwardcurrent of 20 mA (DC) at room temperature. The emission wavelength ofthe LED with a well (barrier) thickness of 8 nm (12.5 nm) has been foundto be 439 nm. It was found that the peak wavelength appeared to increaseby increasing the barrier thickness from 12.5 nm to 27.5 nm. The peakemission wavelengths of the LEDs with barrier thicknesses of 12.5 nm, 20nm, and 27.5 nm were 439 nm, 455 nm, and 483 nm, respectively. The peakemission wavelength of the LEDs with a barrier thickness of 27.5 nm orgreater was approximately 480 nm. Thus, it is possible to obtain longwavelength emission via thick barrier MQWs, with a barrier thickness of27.5 nm or greater, on m-plane substrates.

FIG. 3 shows the EL spectra of the LED with a barrier thickness of 37.5nm, for various injection currents. It was found that all spectra showeda single peak emission wavelength around 475 nm.

The EL intensity and peak wavelength as a function of injection currentis shown in FIG. 4. The peak wavelengths at 1 mA and 100 mA were 478.4nm and 476.1 nm, respectively, indicating that only a 2.3 nm blue shiftwas observed. This suggests that the polarization is reduced.

FIG. 3 and FIG. 4 also show how the quantum wells 102 may be capable ofemitting light 118, having a peak wavelength, in response to a range ofinjection currents passing through the film 100; and the quantum wellsmay have a nonpolar orientation such that interfaces 130 a, 130 bbetween the quantum wells 102 and the barrier thickness 106 a, 106 b,and interfaces 132 between quantum well 102 periods, are nonpolarplanes, wherein the nonpolar plane(s) and/or an alloy composition of theactive layer (e.g. quantum well 102 and/or barriers 104 a, 104 b) may beselected to reduce the polarization of the film 100 so that the peakwavelength remains significantly constant (for example, but not limitedto, within 2.3 nm of the peak wavelength) for a range of injectioncurrents, thereby defining a degree of nonpolarity of the quantum wells102 or the film 100. The range of currents may produce a range ofintensities of the light emitted, wherein the maximum intensity is atleast 37 times the minimum intensity, for example (i.e. the maximumcurrent in the range of currents creates a maximum intensity which is atleast 37 times the minimum intensity created by the minimum current inthe range of currents). However, the range of currents and intensitiesis not limited to a particular range.

Possible Modifications and Variations

In addition to the m-plane GaN free-standing substrates described above,an a-plane substrate, or miscut substrates of GaN, or other foreignsubstrates, such as c-Al2O3, r-Al2O3, m-plane SiC, ZnO, and γ-LiAlO2,can be used as a starting material as well. Any substrate suitable forthe growth of nonpolar III-nitride compounds may be used.

Although the present invention has been demonstrated using InGaN/GaNfilms, AlN, InN or any related alloy (e.g., Group-III nitride compounds)can be used as well.

The present invention is not limited to the MOCVD epitaxial growthmethod described above, but may also use other crystal growth methods,such as HVPE, MBE, etc.

In addition, one skilled in this art would recognize that thesetechniques, processes, materials, etc., would also apply to an a-planesubstrate, miscut substrates in other directions, such as the c-axisdirection, c-plane substrate, with similar results.

The present invention is not limited to a particular range of barrierthicknesses 106 a, 106 b or well thicknesses 108, so long as increasingthe barrier thickness 106 a, 106 b increases the peak wavelength of thelight emission 118 from the quantum wells 102 beyond 400 nm. A devicemay be fabricated using the film 100, for example, the film 100,specifically the quantum wells 102 clad by barriers 104 a, 104 b, may bean active layer of a light emitting device. The device structure is notlimited to a particular light emitting device structure, for example,the device may be an LED and the film 100 may further comprise theactive layer between an n-type layer and a p-type layer for powering theactive layer, wherein the active layer may be an indium containingIII-nitride quantum well 102 sandwiched between two barrier layers 104a, 104 b, a thickness 106 a, 106 b of each of the barrier layers 104 a,104 b may be selected to obtain light emission 118 from the quantumwells 102 with a peak wavelength longer than 400 nm or 475 nm, forexample. For example, the barrier thickness 106 a, 106 b may be selectedto be 10 nm or greater to achieve the light emission 118 with the peakwavelength greater than 425 nm, wherein increasing the barrier thickness106 a, 106 b beyond 10 nm increases the peak wavelength. Thus,increasing the barrier thickness 106 a, 106 b may increase the emissionwavelength. The group III-nitride film 100 may be an active layer of alight emitting device.

Advantages and Improvements

Prior to the present invention, the emission wavelength of InGaN/GaN MQWgrown on on-axis m-plane GaN epitaxial layers was limited toapproximately 400 nm. By controlling the barrier thickness of the MQWs,long wavelength emission from the structures can be achieved.

For example, blue, green, yellow, and white LEDs without polarizationeffects would enhance the devices' performance. Moreover, the presentinvention would enhance the performance of any device.

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A light emitting device, comprising: a nonpolarIII-nitride light emitting device including indium-containing nonpolarIII-nitride multiple quantum wells, wherein an emission wavelength ofthe indium-containing nonpolar III-nitride multiple quantum wells has apeak wavelength of 455 nm or greater for a barrier thickness of theindium-containing nonpolar III-nitride multiple quantum wells that is 20nm or greater.
 2. The device of claim 1, wherein quantum wells andbarriers in the multiple quantum wells have a nonpolar orientation, suchthat interfaces between the quantum wells and the barriers are nonpolarplanes.
 3. The device of claim 1, wherein quantum wells in the multiplequantum wells are InGaN quantum wells and barriers in the multiplequantum wells are GaN barriers.
 4. The device of claim 1, whereinquantum wells in the multiple quantum wells have a thickness of 8 nm orgreater.
 5. The device of claim 1, wherein the peak wavelength remainssubstantially constant for a range of injection currents.
 6. A methodfor fabricating a light emitting device, comprising: fabricating anonpolar III-nitride light emitting device including indium-containingnonpolar III-nitride multiple quantum wells, wherein an emissionwavelength of the indium-containing nonpolar III-nitride multiplequantum wells has a peak wavelength of 455 nm or greater for a barrierthickness of the indium-containing nonpolar III-nitride multiple quantumwells that is 20 nm or greater.
 7. The method of claim 6, whereinquantum wells and barriers in the multiple quantum wells have a nonpolarorientation, such that interfaces between the quantum wells and thebarriers are nonpolar planes.
 8. The method of claim 6, wherein quantumwells in the multiple quantum wells are InGaN quantum wells and barriersin the multiple quantum wells are GaN barriers.
 9. The method of claim6, wherein quantum wells in the multiple quantum wells have a thicknessof 8 nm or greater.
 10. The method of claim 6, wherein the peakwavelength remains substantially constant for a range of injectioncurrents.